In general, an MTI radar is provided with a special system whereby reflected waves from a moving target are distinguished from those from a stationary target. Such a special system will be hereinafter referred to as an MTI processor. This processor is designed to detect reflected waves only from a moving target. More concretely, this processor is designed to delay the received signals of reflected waves by one period, and to compare them with the received signals in the next period.
The basic principle of the MTI processor will be described:
Referring to FIG. 1, which shows a vector diagram exemplifying the basic operation of an MTI processor in the MTI radar, the received signal from a stationary target, that is, clutter is represented by a vector C, and that from a moving target is represented by a vector Ma. When the stationary and the moving target are simultaneously input to the radar receiver, the first received signal becomes equal to the vector sum of the vectors C and Ma, which is represented by a vector A. When the period shifts to the next and comes into the second reception, a signal from the moving target changes into Mb, and therefore, the received signal is represented by a vector B. In the MTI processors the common practice is to delay a received signal A by one period of repetition of radar transmission, and subtract it from the second received signal B. At this time each received signal contains the components of the moving target, and therefore, each vector is liable to variation. As a result, a vector difference M is obtained, which is detected as the moving target. If all received signals are from stationary targets, they are signals without any variation superimposed thereon, thereby causing them to be completely erased, and keeping the output at zero.
Reference will be made to FIG. 2, which shows a block diagram of an MTI radar known in the art, which employs the MTI processor described above:
The illustrated circuit is designed to obtain a vector difference between the received signals, wherein the processing is conducted twice under a double erasing structure. The operation of the processor will be described in detail:
The output of a reference signal generator 1 is divided into two; one is kept in the same phase as the output of the generator 1 per se, which is input to the phase sensitive detecter (PSD) 2, and the other is phase-shifted by 90.degree. by a 90.degree.-phase shifter 3, which is input to a phase sensitive detecter 2a, where the received signal is phasically detected by comparison with the reference signal right-angularly displaced therefrom. This means that the input signal Si is divided into two orthogonal components by the two phase sensitive detecters 2 and 2a. The output of the phase sensitive detecter 2 is an X-axis component, commonly called I channel component. Likewise, the output of the phase sensitive detecter 2a is a Y-axis component, commonly called Q channel component. By using these components the vector arithmetic operation is conducted.
The analog outputs of the phase sensitive detecters 2 and 2a are digitized by A/D converters 4 and 4a. In this way they are decomposed into X- and Y-axis components. The first received signal after digitization is delayed by one period through delay lines 5 and 5a, and when the second received signal is input, arithmetic operation is performed so as to obtain a vector difference by the subtracters 6 and 6a. The outputs of the subtracters 6 and 6a are subjected to the second arithmetic operation by delay lines 5b, 5c and subtracters 6b, 6c so as to obtain a further vector difference. The outputs X and Y of the subtracters 6b and 6c are respectively input to multipliers 7 and 7a, thereby obtaining X.sup.2 and Y.sup.2. Then the sum of X.sup.2 and Y.sup.2 is operated by means of an arithmetic unit 8, thereby determining the amplitude of the vector of a remainder after the double erasing. The output obtained through the MTI processing is converted into analog by an D/A converter 9, and is output to the outside.
FIG. 3 shows a vector diagram exemplifying the operation of the double erasing. An output after the double erasing is obtained as a difference between the output M.sub.1 and M.sub.2 after the single erasing, and it becomes as the following: EQU M.sub.3 =(M.sub.2 -M.sub.1)/2
The output after the single erasing, indicated by .vertline.M.sub.1 .vertline., is well known as MTI filtering characteristic. It takes a sin waveform, which is expressed by the following formula: EQU .vertline.M.sub.1 .vertline..alpha..vertline.sin .pi.Tfd.vertline. . . . (1)
wherein
T: period of repetition of radar transmission; PA1 fd: Doppler frequency PA1 a single erasing circuit for erasing a vector signal from a stationary target among all the input vector signals including reflected waves both from a moving and a stationary target; and PA1 an arithmetic circuit for removing any other variations than ones from the moving target by calculating the inner product of the post-single-erasing vector signal and the pre-erasing vector signal.
The output after the double erasing: .vertline.M.sub.3 .vertline. is expressed by the following formula: EQU .vertline.M.sub.3 .vertline..alpha..vertline.sin .pi.Tfd.vertline..sup.2 . . . (2)
It must be noted that the characteristic expressed by the formula (2) is only applicable to rotating vector signals as represented by a vector signal of a moving target, and accordingly that a larger portion of a signal caused by the moving target will be erased as the erasing becomes multiple. To the contrary, other variations than those due to the moving target will not become rotating vectors, but become straight vectors as shown in FIG. 4. Accordingly, erasing effect does not increase through the multiple erasing with respect to these variations, and a large portion of them remain unerased.
FIG. 4 shows one of the examples mentioned above. The vector A is a first received signal, the vector B is a second received signal, and the vector D is a third received signal, wherein the vectors A and D are equal. Such a signal D does not occur in cases of moving target, but it is likely to arise in a case that the variation is caused by a ripple at a power source. In this case the output obtained through the double erasing becomes a signal .vertline.sin .pi.Tfd.vertline. of the same amplitude as the output after the single erasing, which means that the double erasing had completely no effect.
The prior art MTI radar is constructed in the aforementioned manner, and consequently, if the signal undergoes any change, a larger portion of the signal caused by the moving target is erased than those caused by variations. To prevent this, it is essential that a received signal from a stationary target undergoes no change or variations. In order to achieve this the transmitting signal must be stable. To this end the radar system is free from any impurities, such as ripples at the power source, which are types likely to cause change or variations in the transmitting signals. This naturally leads to an increased size of the system, and complicated circuitry.